David Miliband has delivered a positive assessment of his brother's prospects of becoming prime minister.
The former foreign secretary insisted Labour "can win" the next general election under Ed's leadership.
But he also warned the party against believing it can "default into power" as the country's economic woes continue.
The comments came in a speech this evening hosted by House of Commons Speaker John Bercow.
Mr Miliband, who stepped down from the Labour front bench after losing the leadership race two years ago, said politics became "much more uncertain" in times of crisis.
"Younger listeners may not know this, but governments can actually lose elections before they win three in a row," he said.
"In the 1970s there were four prime ministers and five governments in nine years.
"For me and my party, this is great news. In 2015 Labour can win the general election and Ed can be in Downing Street."
However, he went on: "My own sense is that the Labour leadership are right to warn against trying to default into power.
"Reassuring ourselves that all is well and that if we hold tight to old verities the electorate will realise the error of their ways - and there is always a market for this in both main parties - is the most dangerous plan of all.
"Voters in a time of crisis can swing from government to opposition, that is true, but they can also desert the mainstream parties.
"And crises produce a flight to safety in politics as well as in financial markets.